Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel. However, leaks in the emissions control system can inadvertently allow fuel vapor to escape to the atmosphere. Thus, various approaches are used to identify such leaks.
One example leak detection method is described by Hassdenteufel et al. in U.S. Pat. No. 7,073,376. Therein, during engine operation, fuel systems leaks are detected by applying either a negative pressure or a positive pressure on the fuel system. In particular, the fuel tank is either over-pressurized by applying positive pressure from an air pump, or under-pressurized by applying an engine intake vacuum. Based on a change in the fuel tank pressure, relative to a pressure change obtained across a reference leak/orifice, leak detection is determined. In still other approaches a vacuum pump may be used to apply a negative pressure on the fuel system for leak detection.
However, the inventors herein have identified potential issues with such an approach. To perform the leak detection routine, an air pump or a vacuum pump is operated. As such, operation of the pump may consume vehicle power and reduce fuel economy. In addition, the need for a dedicated pump increases component costs. As another example, some leaks may be masked in the presence of positive pressure while others may be masked in the presence of negative pressure. If the leak goes undetected, exhaust emissions may be degraded.
In one example, the above issues may be at least partly addressed by a method for a boosted engine. The method comprises, indicating fuel system degradation in response to a change in fuel system pressure following application of each of a positive pressure generated at a turbocharger and a negative pressure generated at an engine intake. In this way, a pump-less system is provided wherein existing engine turbocharger hardware is used to perform an engine leak test.
For example, when leak detection conditions are met while the engine is operating with boost, a positive pressure leak test may be performed. Therein, each of a regulator valve and a canister purge valve may be opened to draw a portion of the boosted intake air (compressed by a turbocharger compressor) and apply it on a fuel tank via a canister. After applying the positive pressure for a duration (e.g., until a target fuel tank pressure has been achieved), the applying of positive pressure may be discontinued, and a change in the fuel tank pressure may be monitored. If the fuel tank pressure falls from the target pressure to atmospheric pressure at a fast rate (e.g., higher than a threshold rate), then it may be determined that a leak is present in the fuel system.
However, even if the fuel tank pressure falls at a slow rate, a leak may be present but may be masked by the positive pressure. Thus, to confirm the presence of no leaks, a negative pressure leak test may also be performed. Therein, the canister purge valve may be opened to draw a portion of the engine intake manifold vacuum and apply it on the fuel tank via the canister. After applying the negative pressure for a duration (e.g., until a target fuel tank vacuum has been achieved), the applying of negative pressure may be discontinued, and a change in the fuel tank vacuum may be monitored. If the fuel tank vacuum rises from the target vacuum to atmospheric pressure at a fast rate (e.g., higher than a threshold rate), then it may be determined that a leak is present in the fuel system. In other words, positive boost pressure is opportunistically used during boosted engine operation to perform a positive pressure leak test while natural engine vacuum is opportunistically used during naturally aspirated engine vacuum conditions to perform a negative pressure leak test.
In this way, positive pressure from an existing engine turbocharger can be used to perform a positive pressure leak test. By using the boosted intake air generated by the turbocharger compressor to perform the leak test, existing hardware may be used and the need for a dedicated positive pressure pump is reduced. As such, this provides component and cost reduction benefits. By using both positive and negative pressure to determine fuel system degradation, leaks masked by the presence of positive pressure can be identified by the negative pressure leak test, while leaks masked by the presence of negative pressure can be identified by the positive pressure leak test. By improving leak detection, exhaust emissions can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.